1. Field of the Invention
The present invention relates to a variable damping force type shock absorber able to detect and adjust a damping force generated therein, and a control system for controlling the shock absorber.
2. Description of the Related Art
Preferably, the damping force of a shock absorber is changed according to the state of the road or motion (running state) of the automobile to give a comfortable ride to passengers, etc., in the vehicle. Japanese Unexamined Patent Publication (Kokai) No. 58-194609 discloses a shock absorber capable of changing the magnitude of the damping force. This shock absorber comprises a cylinder, a piston inserted in the cylinder to separate an inner space of the cylinder into two oil chambers communicated by a orifice formed in the piston, and a rotary valve for regulating the magnitude of the cross-sectional area of the orifice, and is capable of regulating the magnitude of the damping force by changing the cross-sectional area by means of the rotary valve.
In the prior art system for regulating the damping force of the shock absorber in accordance with the state of the road or motion of the vehicle, a plurality of sensors are installed in the car; such as a sensor for detecting the rotation angle of the steering wheel, a throttle sensor, a brake sensor, or an ultrasonic sensor for detecting the roughness of the road are used to detect or forecast the state of the road or the motion of the vehicle indirectly, and the damping force regulation is carried out according to the result of the forecast.
This prior art system has several disadvantages, in that the cost of system is increased because of the various types of sensor required, the constitution of a circuit for forecasting the motion of the vehicle on the basis of the detection signals received from a plurality of the sensors becomes intricate, and the sensitivity of the detection of the state of the road by the ultra-sonic sensor, etc., is limited due to mud-splashing, etc.
To solve the above described problems, a method has been disclosed wherein the road state and the motion of the vehicle are detected on the basis of the movements of four shock absorbers installed at the front and rear wheels respectively. This method distinguishes each of the above states on the basis of the characteristic movements shown by the four shock absorbers, as described below, in accordance witn the state of the road or the motion of the vehicle.
Namely, in the "squat" phenomenon caused by rapid acceleration from a standstill, the two shock absorbers at the front wheels expand and the two shock absorbers at the rear wheels contract. In the "nose-dive" phenomenon caused by a sudden braking action, the two shock absorbers at the front wheels contract and the two shock absorbers at the rear wheels expand. Further, in the "rolling" phenomenon caused by a sharp or sudden turn to the right or left, the shock absorbers on one side expand and the shock absorbers on the other side contract. When the vehicle is running on a rough road surface, each of the four shock absorber expands and contracts at random. Therefore, by detecting the expansion and the contraction of the four shock absorbers and the mutual relationship therebetween, it is possible to detect the motion of the vehicle and the state of the road. Further, on the basis of this information, it is possible to regulate the damping force of the four shock absorbers individually, to bring about an optimum running state in accordance with the state of the road and the motion of the vehicle.
In the control of the damping force, for example, in order to control the damping force in accordance with the results of detection of the phenomena such as "squat", "nose-diving", or "rolling", it is necessary to detect these phenomena at an early stage to properly control the damping force, otherwise a sufficient effect cannot be attained. A method for detecting the expansion and contraction of the shock absorber has been proposed, in which the expansion and contraction length of the shock absorber is directly measured by means of an expansion and contraction sensor. Japanese Unexamined Utility Model Publication (Kokai) No. 60-23334 discloses such an expansion and contraction sensor.
However, if this prior art sensor is used to detect the phenomena such as "squat", "nose-diving", and "rolling", the decision of whether or not that phenomenon is severe enough to necessitate a damping force control or is so insignificant that the damping control is not needed cannot be made until the phenomenon is in progress. Accordingly, it is difficult to execute an effective damping force control by means of the prior art expansion and contraction sensor.
To solve this problem, a method has been proposed that is based on the rapidity of the expansion and contraction. In this method, the differential coefficient with respect to the time of the expansion and contraction of the shock absorber is calculated to obtain the speed of the expansion and contraction, and it is decided at an early stage whether or not the phenomenon is severe. However, in this method, a process for differentiating the output of the expansion and contraction sensor with respect to the time is necessary, and accordingly, a processing circuit must be included to accomplish this differentiation. Further, the expansion and contraction sensor that must be adopted in this method is very intricate.